These days, a photon counting computed tomography (CT) device is known in which a detector that implements the photon counting technique is used. Unlike an integral-type detector, a detector implementing the photon counting technique outputs signals that enable individual counting of X-ray photons that have passed through a test subject. Hence, in a photon counting CT device, it becomes possible to reconstruct X-ray CT images having a high signal-to-noise ratio (SN ratio).
Besides, the signals output by a detector implementing the photon counting technique can be used in measuring (differentiating) the energy of the X-ray photons. Hence, in a photon counting CT device, imaging can be done by dividing projection data, which is collected by bombarding X-rays of one type of X-ray tube voltage, into a plurality of energy components.
As a detector implementing the photon counting technique, an “indirect-conversion-type detector” is known in which the incident X-ray photons are temporarily converted into a visible light (a scintillator light) using a scintillator and then the scintillator light is converted into electrical signals (an electrical charge) using an optical sensor such as a photomultiplier tube. Herein, the optical sensor individually detects each scintillator photon obtained by conversion of radiation by the scintillator, and then detects the radiation falling on the scintillator and measures the energy of that radiation.
In recent years, development of silicon-based photomultipliers is being actively pursued. Moreover, there has been development regarding weak-light detection systems such as a detector that implements the photon counting technique using a scintillator and a photomultiplier. In that regard, further development is being carried out with the aim of enhancing the performance.
In a conventional weak-light detection system, the electrical charge that is output from a photomultiplier is integrated by an integrated circuit for a predetermined period of time and is converted into a voltage. Then, the voltage is subjected to sampling/holding and AD conversion (analog-to-digital conversion) in that order. Subsequently, digital signal processing is performed with respect to the obtained digital signals and a histogram is created.
In such a conventional weak-light detection system, the count rate of the electrical charge (i.e., the count rate of the scintillation photons) is limited due to the AD conversion time. In order to achieve a high count rate, it is desirable to implement a high-speed AD conversion method such as the flash method. However, since a high-speed AD conversion method such as the flash method requires a lot of comparators, it leads to an increase in the circuit area and the power consumption. Hence, in the present situation, it is difficult to implement a high-speed AD conversion method.
Particularly, in a photon counting CT device, the count rate of the X-rays falling on a scintillator is estimated to be, for example, about 108 cps (counts per second). For that reason, there is a demand for the development of a reading circuit capable of measuring high-speed and high-energy data at a high resolution in a simultaneous manner in a few hundred channels. However, as described earlier, a high-speed AD conversion method such as the flash method requires a lot of comparators, thereby leading to an increase in the circuit area and the power consumption. Hence, it is difficult to implement a high-speed AD conversion method. Consequently, using the currently available reading circuits, it is difficult to perform measurement in a simultaneous manner in a few hundred channels.